Method of manufacturing electrical resistances and articles resulting therefrom



Jan. 30, 1962 J. M. N. HANLET 3,019,137

METHOD OF MANUFACTURING ELE CTRICAL RESISTANCES AND ARTICLES RESULTINGTHEREFROM Filed Jan. 25. 1957 CARR/El? 6A5 SUPPLY ATTORNEYS UnitedStates Patent Ofiice 3,@19,l3? .Patented Jan. 30, 1952 METHOD OFMANUFACTURING ELECTRICAL RESISTANCES AND ARTICLES RESULTING THEREFROMJacques Marie Noel Hanlet, Paris, France, assignor to SocietedElectronique et dAutomatisme, Courbevoie, France Filed Jan. 25, 1957,Ser. No. 636,410 Claims priority, application France Feb. 11, 1956 3Claims. (Cl. 117212) The present invention relates to an improved methodfor manufacturing thin electrically conducting films having a highspecific resistivity and a predetermined temperature coefficient, whichfilms are homogeneous and strongly adhere to the surface of aninsulating base plate therefor. The meaning intended for the word thinthroughout the present specification and claims must be understood asrelating to films of 'a thickness less than or at most equal to onemicron.

It also relates to such improved resistive films that may result fromthe said method of manufacturing.

A further object of the invention is to so conduct the said method ofmanufacturing that the products resulting therefrom present a highquality of surface state, chiefly a very high melting point, a goodoptical polish and a quite high resistance to mechanical abrasion (thetwo last points being specially important when such films are intendedfor sliding contact potentiometer structures.

From a general point of View, a method according to the invention ismainly characterised in that a homogeneous mixture of at least twohalides having a predetermined ratio of components is first prepared.The said mixture is then vaporized and brought to a pyrolytic conversionchamber for the formation therein of a film of the products ofdissociation of the said halides onto a fiat and heated surface, ofdielectric material. A chemical conversion of the said elements into acomplex of oxides and nitrides is ensured for obtaining the finalelectro-conduc tive films constituting the body of the resistanceproper.

Referring to the attached drawing, the said method of manufacturing anelectroconductive film according to the invention, may be explained asfollows:

A homogeneous mixture of halides of the required elements is placed intoa carrier 1 within a vessel 2 which is uniformly heated by means of aheating envelope 3. The

temperature is controlled from a thermometer 8 by a heat or temperatureregulator 9 fed for instance at 27 by an A.C. current. One end of thevessel 2 is provided with an obturator and at the other end thereof, thesaid vessel is opened upon an insulated pipe 4 of substantial crosssection in order to avoid losses in the gaseous stream issuingtherefrom.

A gas is supplied from a container 7 to the vessel 2 through a regulatedfeeder 6. The evaporated halides will be carried therein through thepipe 4 to a vessel 10 constituted by a bell-shaped wall 11 connected byan airtight joint to a base plate 12. An outlet 13 is provided for thisvessel through which the excess of gas and vapours may pass to adabbling or moistening equipment and including a water pump 15 which,fed with water through a pipe 16, sucks through 14 the remaining gas andvapours and evacuates it with the aid of a ventilator 17.

An axle 18 passes through an air-tight joint into the vessel 10 and isdriven at a low speed from a motor 19. At the end of the said axle 18 issupported a heat-retaining mass 20 subjected to the action of a highfrequency heater 23 fed from an electrical oven source 24. The heatingis controlled from an optical pyrometer 25 and a supply regulator 26. Anasbestos screen 22 ensures the protection of the wall 11 against theheating. Mass 20 is supported on a plate 21 of dielectric material such,for

instance as a high melting point glass. This plate 21 constitutes thesupporting base for the film to be deposited thereon. The heating massis thermally matched to the said plate 21 in that the cooling thereof iseffected, after the heating current is turned oil, during an interval oftime Which is sufficient to prevent any damaging strain within the plate21.

A capillary pipe 28 passes coaxially through the pipe 4 and is providedwith a cock 33 for a controlled admission therethrough within the vessel10, when required, of a limited quantity of a gas generated within avessel 30. This vessel contains for instance a solution of ammonia. Thegeneration of ammoniacal gas is controlled therefrom by means of heatingmeans 31 provided with thermometric regulating means 32. As it will beherein after explained, an admission of such a gas will be useful whennitridation of the films resulting from pyrolytic reactions of halideswithin the said vessel 10 is required, either for a surface treatment orfor a complete nitridation process. Oxides will be obtained when'thecarrier gas is made of or comprises oxygen, and nitrides will beobtained by using an inert carrier gas, such as nitrogen, along with NHas the nitridation reagent. In certain cases however, no carrier gas isrequired. In such a case, the part of the equipment including thecontainer 7 and associated components may be omitted.

The components of the mixture in the evaporator consist of halides (orsalts of halogen) of the elements, viz. metalsand/or metalloids, theoxides and/or nitrides of which are finally produced, as such componentsonly require quite low temperatures for their fusion and vapourproduction. In any case however, the decomposition temperatures of thecomponents will always be much higher than the temperature of the plate21 which is required for the pyrolytic reaction at 10. Such a pyrolyticreaction is too well known per se to be herein detained. It consists ofthe dissociation of a volatile complex of an element, in the vapourphase thereof, at a temperature much lower than the melting point of theelement, metal or metalloid,included therein. The com posite vapoursbrought into the vessel 10 dissociate and result in the formation uponthe surface 21 of a very adherent deposition, which cannot be evaporatedeven at very high temperatures. The nature of thethus obtained filmapparently depends upon the initial components of the evaporated mixtureand also upon the atmosphere wherein the pyrolytic reaction is made.

In such a reaction, the control of the temperature of the receivingplate is one of the methods which may be used for controlling theresulting surface state of the film, i.e. the fineness of the film,provided the pressure of the vapour atmosphere is made constant duringthe said reaction. Varying these two conditions, it is quite possible toobtain a film of very fine structure with a low temperature of operationand conversely. a structure of coarse granulometry with a highertemperature of operation, thoughlower than the temperature ofdissociation of the initial. mixture. An increase of the density of thevapours or an increase of the pressure within the reaction chamber, oran increase of both these conditions, permits the obtaining of films,the surface structure of which is a crystalline for practicing theinvention, it has been found of advantage to operate the pyrolyticreaction at the highest admissible temperature and to determine thefineness of the film by adjusting the pressure and/or the density of thevapoursgiving rise to such a film.

But the invention enables the production of thin resistive films havinga predetermined temperature coefficients. It is well known that a greatmany metallic oxides present negative temperature coefiicients. Nitridesand very fine one. Actually,

generally present positive temperature coefiicients. As the filmsaccording to the invention are composite ones, wherein the componentsthereof are quite intimately mixed on a substantially molecular scale,it will be possible to provide therein either oxides of the P type ofconduction, having an excess of electrons, such for instance as V ZnO,W0 TiO Ta O or oxides of the N type of conduction, having a lack inelectrons, such for instance as CuO, NiO. It is apparent that associations including oxides of both these types of conduction may easily beprovided with predetermined ratios, in films produced according to thepresent invention.

It is also quite easy to provide associations of oxides and/or nitridesin such a method of processing as herein above defined in order toobtain electroconductive films of predetermined resistivities.

For instance and with a view of illustrating the above, one may considerthat the initial mixture comprises stannic bromide and niobiumpentabromide. Within an oxidising atmosphere, this mixture will give,through pyrolytic reaction, a composite film of stannic oxide andniobium pentoxide. Bromine will remain in the vapour state, and beevacuated in such a state. Thereafter introducing NH within the vesselniobium nitride is obtained in the said film. There is no isomorphismbetween stannic oxide and niobium nitride, but the mechanical hardnessof both are similar and the surface hardness of the resulting filmthereof will actually be of the same order as that of vanadium. Themelting point of the complex constituted by tin oxide and niobiumnitride will have a value of about 1700 C. The tetravalent tin oxide hasa coefficient of temperature which is highly negative until 700 C. andsuddenly reverses at this point. The niobium nitride presents a reverseproperty in that it becomes superconducting at the neighborhood of 10 K.Their respective resistivities are, at normal temperature, equal to22.10 ohms/cm. for the niobium nitride and to about 10. ohms/cm. for thesaid tin oxide. By regulating the ratio of the components in the initialmixture and consequently in the finally produced film, a predeterminedvalue of coefficient of temperature and electrical resistivity may beobtained.

Instead of tin, other elements from group IV may advantageously also beused, mainly silicon and titanium, and the impurities willadvantageously be of the third and fifth groups according to the rangeof resistance and temperature coefiicient values which is required forthe resulting films.

One example of a resulting complex which will give resistances of a highdegree of stability, a high melting point therefor, and also a very goodresistance to abrasion, from the mechanical point of view, is thatwherein are included titanium and/ or tantalum nitrides. Silicon mayalso be used instead of titanium, sometimes with some advantage due tothe isomorphism thereof. But of course, quite a number of other elementsmay be incorporated in resistive films according to the invention.

Considering for instance two practical cases of utilisation of theequipment which has been herein above described:

In the first case, a homogeneous mixture is made which includes thefollowing proportions by weight of 91% of tin bromide (SnBr 3.5% ofniobium pentabromide (NbBr 5.4% of bismuth tribromide (BiBr and the saidmixture is introduced Within the vessel 2, the temperature of which ismaintained between 215 and 218 C.

The speed of rotation of the axle 118 is between one and .2 rotation persecond at least. The temperature of the plate 21 is established at 500C. 120 C.

Through the vessel 2 a flux of carrying gas is established for thehalide fumes, which comprise two parts in volume of nitrogen and onepart of hydrogen, with a supply rate of .2 litre per minute.Simultaneously a volume of NH is fed at a supply rate of .66 lrtrevperminute through 33.

With such adjustments, and starting from a nnxture weighing 1.8 grams,the film produced over a surface measuring four square centimeters has atotal resistance equal to 40,000 ohms i10%. The temperature coefficientthereof is positive and equal to 510-. The deposition of the film isobtained within three minutes.

The resistance value of a film is approximately a linear and reversefunction of the weight of the initial mixture, and consequently of theweight of the film. Starting from a mixture Weighting 3.6 grams, thefilm coating the same surface of 4 square centimeters will have a resistance value of about 20,000 ohms.

In the second case, the following mixture is established within a verydry atmosphere and at a very low tem perature:

96% per weight of titanium tetrachloride (TiCl 3.5 hafnium tetrachloride(HfCl .5 tantalum pentachloride (TaCl This mixture is placed within thevessel 2 which is first heated to 140 C. in five to six minutes and thenup to 221 C. in about one half or one minute, in order to ensure an evendiffusion of hafnium and tantalum. No introduction of ammoniao gas ismade. The carrier flux is made of or contains oxygen. The result is theforma tion of a complex film including the oxidies of the above elementson the plate 21. For the same weight and surface area as above, theresistance value of this film is equal to 200,000 ohms and thetemperature co efficient is negative and equal to 4.10- up to 200 C.

In the first above case, the carrier gas does not make any part of thepyrolytic reaction process for the forma tion of nitrides. Nitrides aredue to the introduction of ammoniac gas within the vessel 10. If theaddition of NH were cut off, in such a process, the pyrolytic conversion would result in the formation of a film of an alloy of pure tin,niobium and bismuth. Such an alloy might then be submitted to a furtheroperational step for converting the metallic alloy into a nitridecomplex. The same sequence of steps would be effective for thepreparation of oxide films, i.e., first the formation of a film ofmetallic alloy and then the oxidising thereof.

But the better procedure for applying a further conversion step to afilm produced from a pyrolytic conversion appears to be as follows:first the pyrolytic process is made within an oxidising atmosphere, andconsequently the film resulting therefrom is an all oxide complex. As analternative, the process may be carried out to bring a nitridation of atleast part of the film. It must be noted that for halides which arestrongly hydrometric, the first step will always give an oxidising. Insuch cases it will be more economical to use moistened air for thecarrying of the halide fumes, instead of utilising therefor a speciallyprepared gas mixture.

If the specific resistivity of the films is determined by an appropriatechoice and adjustment of the ratio of the components thereof, the totalresistance value of the film depends upon the weight of the materialtherein and the area thereof, it being understood that the thickness isquite uniform at any point of the said area. The resistance R of a filmhaving a thickness s, a length L and a width T, and having a specificresistivity p is R=(P L)/(s T). When the film is made as an annulararea, for establishing therefrom a sliding contact potentiometer, thewidth is equal to the difference of the radii and the length, to that ofthe arc of circumference of average radius. It is apparent that any lawof variation of the resistance along the potentiometer track definedtherefrom may be obtained from a variation of the width thereof infunction of the angle of the radius along 5 which is conducted thisWidth radius. I

In order to obtain a shape of resistive film and an area thereofpresenting a total resistance value which is predetermined andaccurately adjusted, and also a linear change of resistance along thesaid area which is also accurately adjusted, two ways are possible:first, a mask may be formed over the surface of the base plate 21through which the deposition of the film isoperated; secondly thesurface the base plate may be completely coated with the film and partsof the said film are removed where in excess. The second way mayadvantageously be used for further adjusting a resistance obtainedthrough a mask according to the first method.

It is apparent that the said mask must be such that it does not impedethe process of formation of the resistive film. It may be prepared asfollows:

The two following mixtures are separately made:

I. 58% of finely powdered silica, passed through a 325 mesh sieve, 26%of pure alumina, 11% of potassium oxide K 0, 4% of sodium oxide Na O and1% of sodium borate Na B O weight percentages;

II. 73% of sodium sulfosuccinate, 12% of methanol, of glycerin and ofcarboxymethylcellulose, weight percentages.

The mixture I is placed into solution into the mixture Ii until a sirupymixture is obtained of such a viscosity that it may be applied to theplate 21 by spraying, serigraphy or painting, that is to say to allparts of the said plate which are due to remain free from the film to beestablished thereon. After deposition of the said film, the mask iseasily removed from the base plate.

The removal of parts of a resistive film, on the other hand, cannot beeasily made by abrasion or direct mechanical action. Of course diamondtools or electric with respect to a reference sparking may be used butit appears preferable to proceed as follows:

A mixture is prepared comprising within 350 cubic centimeters of waterand 5 cubic centimeters of glycerin, 1 gram of sodium nitrite and .5gram of pyrophosphate of sodium to which is added a value of 2.5 gramsof bentonite. This mixture may be made as follows: a gel is preparedfrom the admixture of bentonite with 100 centicubes of water, anothermixture is separately prepared containing glycerin and the same volumeof water, a third mixture is made containing the pyrophosphate, thenitrite and the remaining water. The two latter mixtures are mixed andthe gel is added thereto.

Once the overall mixture is made, an equal volume of methanol is addedthereto and in the methanolic solution thus obtained, fine powder ofzinc is mixed in the proportion of 1 gram of zinc for 2.5 grams of thesaid solution. The resulting product is sprayed through a mask over theparts of the film to be removed. After drying, a furthcr spraying ismade with a solution in water of hydrochloric acid, the concentration ofwhich is between 1 and 10% per weight of acid. After about ten minutes,a reaction is completed whereby the zinc, of electronegtiveconductivity, reduces in the presence of the acid all parts of the filmsprayed by the above-said solution. Instead of zinc, tin, cadmium, ironor aluminum may be used in the said solution.

The resistances having thus being made and adjusted, electricalconnections must be attached thereto. It will be of advantage not tohave recourse to mechanical connecting means to such films and, ofcourse, once the adjustments are made it is highly desirable not toalter the resistance of the film, so that the electrical connectionsmust present a much lower electrical resistance than the film.

According to an auxiliary feature of the invention, use is made for theestablishment of such connections of the same method as has been hereinabove described. A mask is set over the film and those parts of the baseplate thereof which are due to remain free from any coating. Thereafter,the thus coated base plate and film are placed at the location of thebase plate 21 in the shown equipment or plant, and a further pyrolyticreaction is operated with a material in the vessel 2 which now includesboron bromide together with another bromide such as titanium or othermaterial. The resulting connections are thus made as film strips ofboron and other oxides which are of a far greater conductibility thanthe resistive films previously formed, due to the presence of boron, asis well-known per se.

What is claimed is:

1. A method of producing electrical resistor elements having a highlyaccurate, predetermined resistivity and temperature coefiicient and ahigh degree of surface hardness which comprises mixing into a homogenousstream controlled amounts of vapors of the halides of at least twoseparate chemical elements, one of the components of said streamconstituting at least by weight of the stream and being a halide of anelement selected from the group consisting of tin, silicon and titanium,the remainder of said stream being a minor component selected from thegroup consisting of the halides of niobium, bismuth, hafnium, tantalum,tungsten, zinc, nickel, copper and vanadium, carrying said mixture ofvaporized halides into a gaseous stream comprised of an inert carriergas and a reactive gas selected from the group consisting of watervapor, oxygen and ammonia, and mixtures thereof, contacting theresulting gaseous mixture stream with a dielectric base heated to atemperature high enough pyrolytically to decompose said halides intohalogen and the free element and cause said reactive gas to react withthe base element forming a compound selected from the group consistingof the oxides and nitrides of said free elemens as a uniform depositupon said dielectric base in intimate surface cohesion with said baseand evacuating excess vapors from the zone surrounding said heated base.

2. A method according to claim 1, and wherein prior to the pyrolyticreaction, applying to predetermined portions of the said receiving platea layer constituting a mask impeding the deposition of the said filmover the said portions, and after the deposition of the said filmremoving the said mask therefrom.

3. A method according to claim 1 and wherein, after the formation of thesaid film, the step of coating parts of the film with a layerconstituting a mask impeding a deposition of matter over the said parts,and effecting a new pyrolytic reaction in which an halogenated componentof boron is the main component of the new mixture the fumes of whichenter in the said new pyrolytic process.

4. A method as claimed in claim 1 wherein said gaseous stream at firstcomprises oxygen with no ammonia causing an oxide deposit to be formedand thereafter said gaseous stream comprises ammonia causing the depositto comprise, a least in part, a nitride.

S. Amethod as claimed in claim 1 wherein said reactive gas is oxygenforming a deposit of an oxide and, wherein, thereafter the oxide filmdeposit is subjected at elevated temperature to an atmosphere containingammonia whereby the oxide deposit is at least partially converted intonitride.

6. A method as claimed in claim 1 wherein said gaseous mixture stream iscontacted with said dielectric base which is supported by aheat-regulating mass and which is slowly rotated within a chamber heatedto a temperature high enough pyrolytically to decompose said halides.

7. A method as claimed in claim 1 wherein said halide mixture comprisesat least 90% tin halide and at least part of the remainder of themixture is niobium halide.

8. A method as claimed in claim 1 wherein said halide mixture comprisesat least 90% tin halide and at least part of the remainder of themixture is tantalum halide.

9. A method as claimed in claim 1 wherein said minor component comprisesa mixture of two halides.

10. An electrical resistance comprising a dielectric base plate havingdeposited thereon as a uniform coating in intimate surface cohesion withthe base plate, an electrically conductive layer of a thickness up toone micron consisting of a molecular complex of compounds selected fromthe group consisting of oxides and nitrides, at least 90% by weightbeing a compound of an element selected from the group consisting oftin, silicon and titanium and the remainder of the complex beingcompounds of an element selected from the group consisting of niobium,bismuth, hafnium, tantalum, tungsten, nickel, copper, zinc and vanadium.

11. An electrical resistance as claimed in claim 10 wherein said complexcomprises a mixture of oxides and nitrides.

12. An electrical resistance as claimed in claim 10 wherein said complexconsists predominantly of a mixture of oxides with said layer thereofbeing superficially ,nitridised over its entire exposed area.

13. An electrical resistance according to claim 10 and wherein to thesaid film and on the same base plate there exists at least one pair ofelectrical connection films constituted by a complex of oxides includingmainly a boron oxide therein. 2

References Cited in the file of this patent UNITED STATES PATENTS448,915 Erlwein Mar. 24, 1891 1,497,417 Weber June 10, 1924 1,891,235Lawton et al. Dec. 20, 1932 2,551,341 Scheer et al. May 1, 19512,552,626 Fisher et al. May 15, 1951 2,556,991 Teal June 12, 19512,764,510 Ziegler Sept. 25, 1956 2,784,115 Brinsmaid et al. Mar. 5, 19572,798,140 Kohring July 2, 1957 2,859,132 Novak et al. Nov. 4, 1958

1. A METHOD OF PRODUCING ELECTRICAL RESISTOR ELEMENTS HAVING A HIGHLYACCURATE, PREDETERMINED RESISTIVITY AND TEMPERATURE COEFFICIENT AND AHIGH DEGREE OF SURFACE HARDNESS WHICH COMPRISES MIXING INTO A HOMOGENOUSSTREAM CONTROLLED AMOUNTS OF VAPORS OF THE HALIDES OF AT LEAST TWOSEPARATE CHEMICAL ELEMENTS, ONE IF THE COMPONENTS OF SAID STREAMCONSTITUTING AT LEAST 90% BY WEIGHT OF THE STREAM AND BEING A HALIDE OFAN ELEMENT SELECTED FROM THE GROUP CONSISTING OF TIN, SILICON ANDTITANIUM, THE REMAINDER OF SAID STREAM BEING A MINOR COMPONENT SELECTEDFROM THE GROUP CONSISTING OF THE HALIDES OF NIOBIUM, BISMUTH, HAFNIUM,TANTALUM, TUNGSTEN, ZINC, NICKEL, COPPER AND VANADIUM, CARRYING SAIDMIXTURE OF VAPORIZED HALIDES INTO A GASEOUS STREAM COMPRISED OF AN INERTCARRIER GAS AND A REACTIVE GAS SELECTED FROM THE GROUP CONSISTING OFWATER VAPOR, OXYGEN AND AMMONIA, AND MIXTURES THEREOF, CONTACTING THERESULTING GASEOUS MIXTURE STREAM WITH A DIELECTRIC BASE HEATED TO ATEMPERATURE HIGH ENOUGH PYROLYTICALLY TO DECOMPOSE SAID HALIDES INTOHALOGEN AND THE FREE ELEMENT AND CAUSE SAID REACTIVE GAS